1. Field of the Invention
The present invention relates to a muffler which reduces noise on an intake path or an exhaust path.
2. Description of the Related Art
Mufflers, which reduce noise by changing the resonance frequency in order to be able to reduce noises over a wide range of frequencies, are known.
For example, Japanese Utility Model Application Laid-Open (JP-U) No. 6-58151 discloses a muffler in which a movable wall, which is freely rotatable, is accommodated within a resonance box having a substantially cylindrical peripheral wall. Due to a partitioning plate of the movable wall slidably abutting the inner peripheral surface of the peripheral wall of the resonance box and rotating the movable wall, the length and the like of a neck portion, which is sectioned off and formed by the peripheral wall of the resonance box and the movable wall, is changed.
In such a muffler, the arc-shaped configuration of the inner peripheral surface of the peripheral wall of the resonance box, which configuration corresponds to the length from the center of rotation of the movable wall to the end portion of the partitioning plate, must be formed highly accurately.
Further, this structure presupposes that the end plate (side surface) of the movable wall also contacts the inner surface of the resonance box slidably and airtightly. Therefore, a highly accurate planar surface must be formed over a wide range in correspondence with the inner surface of the resonance box.